WIP: Add documentation site with Material for MkDocs

[EiffL]

Summary

• Adds a full documentation site using Material for MkDocs with mkdocstrings for auto-generated API reference from docstrings
• Includes getting started guides (installation, quick start), notable differences from GalSim, API coverage page, and complete API reference stubs for all implemented modules
• Adds GitHub Actions workflow for building docs on PRs and deploying to GitHub Pages on push to main

What's included

• Home page with feature highlights, quick install, and minimal code example
• Getting Started section: installation (pip, GPU, dev setup) and quick start tutorial with jit/grad/vmap examples
• Notable Differences from GalSim: thorough guide covering immutability, array views, RNG, PyTree registration, control flow/tracing, profile restrictions, numerical precision, and the @implements decorator
• API Coverage page (22.5% of GalSim API)
• API Reference (~30 pages) auto-generated from docstrings via mkdocstrings
• CI workflow (.github/workflows/docs.yml): mkdocs build --strict on PRs, mkdocs gh-deploy on push to main

Known limitations

• API function documentation is incomplete — the @implements decorator copies GalSim's docstrings at runtime, but mkdocstrings doesn't always pick them up cleanly. RST cross-references (:func:, :class:) from GalSim's docstrings render as literal text rather than links.
• Some API pages show minimal content where docstring propagation from GalSim didn't work as expected.

🤖 Generated with Claude Code

[codspeed-hq]

Merging this PR will degrade performance by 35.68%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 4 improved benchmarks
❌ 3 regressed benchmarks
✅ 29 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	WallTime	test_benchmark_spergel_xvalue[run]	1.6 s	2.3 s	-31.09%
⚡	WallTime	test_benchmarks_lanczos_interp[xval-conserve_dc-run]	124.7 µs	82.1 µs	+51.82%
⚡	WallTime	test_benchmark_moffat_init[run]	95.5 µs	66.5 µs	+43.68%
⚡	WallTime	test_benchmarks_lanczos_interp[kval-no_conserve_dc-run]	56 µs	41.5 µs	+34.86%
⚡	WallTime	test_benchmarks_lanczos_interp[xval-no_conserve_dc-run]	113.1 µs	74.1 µs	+52.65%
❌	WallTime	test_benchmark_moffat_conv[run]	187.9 ms	282.4 ms	-33.48%
❌	WallTime	test_benchmark_spergel_conv[run]	159.3 ms	247.6 ms	-35.68%

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_{Comparing documentation (3660b3f) with main (ef2c37a)}

[ismael-mendoza]

I flagged a couple of cases in the examples provided that don't actually work. @EiffL could you modify as needed?

[ismael-mendoza]

Is this really true in all cases? For example, the difference between 32-bit floats and 64-bits float can be an issue for very precise shear measurements no?

[ismael-mendoza]

I think this is basically good to go. The last thing is that there is an issue with the API reference as none of the original GalSim or lax description docstrings are showing up at all. Maybe there is an issue with the implements decorator? (other docstrings do show up)